1. Field of Invention
The present invention relates to rapid thermal processing and bonding of materials using radio frequencies (RF) and microwaves, and more specifically to a method and apparatus for rapid heating of materials using solid state variable frequency RF/microwave sources. The invention further relates to a method for rapid thermal processing of semiconductors and materials using the present RF/microwave apparatus. The present invention also relates to a method of selectively heating portions of materials using the RF/microwave apparatus of the invention, and more specifically to the bonding and sealing of microelectro-mechanical systems (mems) and integrated circuits (IC/microchips) using the present method of RF/microwave rapid and selective heating.
2. Related Art
Advantages of Rapid RF/Microwave Heating
There are many applications for using microwave energy for thermal processing of materials. Microwave heating has an advantage of rapid heating over conventional heating techniques. Conventional heating is slow because the heat is applied to the surface of an object. It takes time to transfer the heat from the surface to the interior of the object through thermal conduction. In contrast, microwave heating is fast because microwave can penetrate into objects and heat the interior and exterior simultaneously.
Microwave-Materials Interaction
The mechanisms of microwave heating are based on the interaction between the electromagnetic field and the targeted materials. There are three forms of interaction in general. namely: (1) materials with low complex dielectric constant are poor microwave absorbers and not good for microwave heating; (2) materials with high complex dielectric constant are strong microwave absorber and very good for microwave heating; and, (3) conductors such as metals reflects microwave so not good for microwave heating in general. However the magnetic filed of microwaves can penetrate a skin depth of the conductor's surface and induce an eddy current to generate heat therein. The skin depth of high conductive metals such as Au, Ag and Cu is about 0.6 to 6 μm within RF/microwave frequency range from 100 MHz to 10 GHz. Therefore when the dimension of a conductor becomes the same scale or less of the skin depth, such as in the cases of the conductive interconnection in IC chips and doped impurity layer in Si wafers, the heating effect of eddy current is significant and the thin conductor layer is no longer a microwave reflector but a strong microwave absorber. The thin layer of a conductor becomes a very good candidate for microwave heating.
Selective Heating
When microwave power radiates on a heterostructure (comprised of different materials), such as a strong microwave absorber and a poor microwave absorber, microwaves will selectively heat up the strong microwave absorber while leaving negligible heating effect on the materials of low microwave absorption.
Methods and Apparatus for Microwave Rapid Heating
It is known that the heating rate of RF/microwave thermal processes depends on the density of RF/microwave energy to be coupled into the materials. Therefore two important conditions for a rapid RF/microwave heating are (1) a high intensity of electromagnetic (EM) field and (2) EM energy to be efficiently coupled into the targeted materials. The methods for coupling EM energy into materials can be characterized as three classes:
1. Capacitive coupling, where energy is coupled into materials via electric E fields;
2. Inductive coupling, where energy is coupled into materials via magnetic H fields: and,
3. Cavity (EH) coupling where energy is coupled into materials via a combination of E and H fields.
Cavity coupling is the most widely used method in microwave heating processes. A microwave heating furnace is typically constructed in the form of either a single mode cavity or a multi-mode cavity operating at a fixed frequency. A single mode cavity can generate a much higher intensity of electromagnetic filed than that of a multi-mode and therefore is more favorable in fast heating processes. Heating rates as high as 10-100° C./sec are achievable using single mode cavity whereas the heating rate in a multi-mode cavity is relative low. There are some technical barriers for further increasing the heating rate to well above 100° C./sec level using cavity techniques used in prior art. First the use of a fixed frequency source leads to a mismatch in resonant frequencies between the RF/microwave source and the loaded cavity when the frequency of the loaded cavity shifts with the temperature changes during a thermal process. Second, the cavity is mechanically tuned so its response to coupling change is slow which results in a slow down of the heating rate.
There are other limitations of cavity coupling techniques in the prior art. One of the significant problems is the arcing and the breakdown of plasma inside the cavity as the input RF/microwave power reaches a threshold level to breakdown the air. Especially in the presence of conductive materials in the cavity, such as metal, the electric field is significantly enhanced at the edge of conductive materials so the arcing may occur at a much lower power level than that of the threshold. The limitation of input power due to the arcing problem significantly limits the ability of using cavity techniques to achieve high heating rate and high heating temperature.
Another limitation of cavity techniques used in the prior art is that the size of the load must be smaller than the size of the cavity. Since the dimension of a single mode cavity decreases with an increase of the operating frequency, the size of the cavity operating at high frequency may not be large enough when the size of load is big, e.g. an 8″ silicon wafer.
Induction coupling method is known in prior art and has been widely used in heating conductive materials. The mechanism of the inductive heating is based on the thermal effect of ‘eddy current’ induced by the magnetic field of an AC current. However, the use of induction heating in prior art is limited in low frequency levels, ranged from 50 Hz to less than 50 MHz. None of the induction heating methods in the prior art has used RF and microwave frequency beyond 100 MHz. Induction heating in not suitable in heating low conductive materials and insulators due to the lack of eddy current.
3. Applications
Rapid Thermal Processing (RTP) of Semiconductors and Materials
An important application of microwave rapid heating techniques is rapid thermal processing (RTP) of advanced materials and semiconductors such as activation annealing of ion implanted wafers for SiC and GaN wide bandgap semiconductors and formation of ultra shallow junction for CMOS devices. High annealing temperature and extremely short annealing time are two critical conditions for these RTP applications. By increasing the annealing temperature while simultaneously reducing the time, high percentage of electrical activation can be achieved and will lead to low sheet resistance. Meanwhile, the short annealing time will minimize the diffusion in CMOS devices to allow ultra shallow junction formation and eliminate surface evaporation in SiC wafers to reduce surface roughness. For instance, the temperature required for post-implant annealing of SiC semiconductor may range from 1500 to 2000° C. along with short annealing time ranged from a few minutes to a few seconds. None of conventional heating methods used in prior art can meet these requirements. Limited by the slow heating rate, conventional furnace heating methods can only perform SiC annealing at temperatures below 1700° C. with annealing time longer than 5 minutes. The relative low annealing temperature of below 1700° C. will result in ineffective recovery of crystal and low activation efficiency. The halogen lamp based commercial RTP systems can only operate at below 1300° C. which makes them incapable for post implant annealing of SiC.
Very high heating and cooling rates ranged from 100° C./sec to well beyond 1000° C./sec are required for RTP of CMOS shallow junction within temperature range of 1150 to 1350° C. The halogen lamp based commercial RTP methods are currently used for RTP of CMOS shallow junction which can operate at heating rate of 100-300° C./sec within annealing temperatures below 1300° C. The best current lamp based RTP systems appear to be viable to shallow junction processes for current 100 nm technology node, although they may be very close to the limit of their capability. However, they will no longer meet the challenges in making ultra-shallow junctions associated with next generation CMOS in sub-65 nm regime or smaller where ultra-high heating rates well above 1000° C./sec are required.
Bonding and Packaging of Mems and IC Devices
The packaging of mems and IC chips generally requires the bonding of the components and the connecting of the internal circuits with external pins. The soldering method is a the most widely used technique for IC and mems packaging where low melting alloys such as Pb—Sn are used as the solder materials. FIG. 1 illustrates a typical example of Flip-Chip solder bonding in connecting the microchip to a substrate where the substrate may be either a print circuit board (PCB) or direct chip attachment (DCA). Two gold pads are first plated on the bonding areas of the silicon chip and substrate. The solder material is then placed between the two pads. By melting the solder to form a metal bump between the pads as shown in FIG. 1, the silicon chip and the substrate are bonded together.
As the demands on miniaturization of IC chips increases, high-density interconnections and a high number of I/O requirements are indispensable in the IC industry. The large space occupied by the solder bumps becomes a limited fact for further shrinkage of the IC package. Another problem with the soldering method is the high residual stress at the bonding interfaces caused by the large coefficient of thermal expansion (CTE) mismatch between the solder bump and the substrates. The high residual stress may induce delamination and cracks in the package during the operation in thermal cycles.
Many mems applications may require special bonding qualities such as high bonding strength, hermetic sealing, chemical resistance and high service temperature. Bonding strength is very important for the mechanical function of many mems. Mems components may also operate under different environment, temperature and vacuum/pressure conditions where hermetic sealing, chemical resistance and high service temperature become the necessary bonding requirements. Low temperature solder bonding can no longer meet these requirements because of its weak mechanical strength, low melting temperature and poor chemical resistance. Other bond techniques and bonding materials are needed. For instance, some noble metals such as Au, Cr, Ni, Pt or glass and ceramics are very good candidates as the bonding materials because of their excellent high temperature properties and chemical resistance. These materials require much higher bonding temperature than that of soldering because of their high melting temperature.
A number of bonding techniques have been developed for using bonding materials other than lower temperature solders, such as Au—Si eutectic brazing, glass-Si fusion bonding and anodic bonding. These bonding techniques have significantly improved the mems bonding quality to meet the special requirements of various applications. However, all these existing bonding techniques are global heating methods so entire mems are exposed to high temperatures for long processing times which may lead to some damages or changes of the microcircuits and microstructure which were already placed on the mems components.
Thus it is important to develop alternative approaches to overcome the shortcomings of the existing techniques for these applications. What is needed is a method for rapid heating up of materials to very high temperatures for RTP of materials and semiconductors. What is also needed is a method for strong bonding and hermetic sealing of mems with bonding materials of high melting temperatures or chemical resistance. What is further needed is a method for small size bonding of IC chips without using solder bumps.